Speckle is a phenomenon created with laser light sources, due to the fact that laser light is coherent. Parallels and synchronized wavefronts simultaneously hit the projection surface. When the light hits the surface, it creates constructive and destructive interference. The first category of interference induces an image deterioration that is often visible by human eye and/or by sensors. In addition to a loss of image quality, visual comfort of the viewer may also be affected.
Several techniques are used in order to remove or reduce speckle. In many cases, light coherence reduction techniques are used. For instance, the light hitting the projection surface is provided from various projection angles. Polarized laser light hitting a depolarized film is also used. Otherwise, illumination using various laser wavelengths may also be used.
Another approach consists in using vibration of the projection surface. The resulting systems are complex, expensive, and involve very specific hardware material.
WO2009/077198 describes an optical system comprising a coherent light source and optical elements for directing light from the source to a target. The optical elements include at least one diffusing element arranged to reduce a coherence volume of light from the source and a variable optical property element. A control system controls the variable optical property element such that different speckle patterns are formed over time at the target with a temporal frequency greater than a temporal resolution of an illumination sensor or an eye of an observer so that speckle contrast ratio in the observed illumination is reduced. The variable optical property element may be a deformable mirror with a vibrating thin plate or film. This solution requires modifying the projection system in order to integrate additional components, such as diffusing elements.
WO2007/112259 describes a system and method for reducing or eliminating speckle when using a coherent light source. A refracting device, comprising a birefringent material, is positioned such that the refracting device intercepts the coherent light. The refracting device rotates, thereby causing the ordinary and/or extraordinary beams to move. The human eye integrates the movement of the beams, reducing or eliminating laser speckle. The refracting device may include one or more optical devices formed of a birefringent material. Wave plates, such as a one-half wave plate, may be inserted between optical devices to cause specific patterns to be generated. Multiple optical devices having a different orientation of the horizontal component of the optical axis may also be used to generate other patterns. Furthermore, the refracting device may include an optical device having multiple sections of differing horizontal components of the optical axis. This solution involves a complex and expensive component, the rotating refracting device. Moreover, the integration of such device requires a specific global design.
Optical MEMS (Micro-Electro Mechanical Systems) are moving structures that are adapted to deflect light over time and space. These structures are usually made of silicon and are operated using different actuation principles including magnetic, electrostatic, piezoelectric and/or thermal.
Classically, MEMS mirrors are used in various optical applications and are usually delivered as stand-alone unprotected chips. When used in scanning applications for example, the incoming light is directly reflected on the mirror and usually does not transmit through any other material or media.
However, an unprotected chip makes the mirror surface subject to optical and mechanical degradations due to dust or other material deposition. Fabrication of unpackaged mirrors may also reduce the fabrication yield of such device due to its sensitivity to external handling and tooling processes. Therefore a packaged MEMS mirror is strongly recommended to obtain high quality mirrors and a high fabrication yield. Among packaged MEMS mirror technologies, wafer-level packaging technology is the most suited for high volume high yield manufacturing.
However when a mirror is protected, or encapsulated, with transparent or semi-transparent windows, if light is passing through the window, a light reflection will occur at both air-window interfaces. These reflections are usually considered as parasitic reflections. A standard way to reduce these reflections is the deposition of anti-reflective coatings on both sides of the window, enabling the reduction of the parasitic reflections down to approximately 0.1% of the incoming light intensity 300 (FIG. 3) if the coating is designed for a single wavelength, and down to 0.3% to 0.4% if the coating is designed for a larger wavelength spectrum such as the entire visible light (430-670 nm).
However, when using a high power light source, such values of parasitic reflection may result in a strong degradation of the reflected light homogeneity. Indeed, as an example, for a laser-based MEMS scanning mirror projection system with a resolution of 640×480 pixels, a parasitic reflection as low as 0.3%, for a coated air-window interface, it will result in a fix parasitic pixel-light spot with a light intensity 1000 times stronger than any other pixel on the projected image or video.
A consequence of such parasitic reflection is that the user will experience a brighter fix light spot in the projection field, which is a clear showstopper for standard use of the device and for customer adoption of the device.
U.S. Pat. No. 6,962,419 describes a package for micro-mirror elements having a window that is not parallel to the substrate upon which the micro-mirrors are formed. Such configuration enables the reflected light to be oriented outside from the projection zone. However, this arrangement does not reduce perceived speckle by a user.
Thus, there is a need for a novel micro-projection system with reduced speckle having MEMS micro-mirrors and MEMS components in general, that do not present the above mentioned drawbacks, namely the complexity and costs problems caused by using specific configurations with additional components used only for speckle reduction. There is also a need for a system avoiding undesired parasitic reflection of the light on the protection window.